The finding highlights the need for having carbon monoxide alarms in your home, since even checking your own appliances won't guarantee that the lethal gas might not seep through your walls from another source, experts say.

"What this study tells me is that carbon monoxide does not stay put in a building, that the barriers between apartments or condos will slow down carbon monoxide, but do not stop it," said Dr. Eric Lavonas, associate director of the Rocky Mountain Poison and Drug Center in Denver.

"Therefore, the best way to protect your family is to have a working carbon monoxide alarm in your home," according to Lavonas, who was not involved with the study.

Carbon monoxide is a colorless, odorless gas found in car exhaust and in fumes from fuel-burning sources such as generators, charcoal grills, gas stoves and wood fireplaces.

"Any source of combustion produces carbon monoxide of some degree, no matter how clean-burning your appliances are," said study author Dr. Neil Hampson, with the Center for Hyperbaric Medicine at the Virginia Mason Medical Center in Seattle.

The report is published in the Aug. 21 issue of the Journal of the American Medical Association.

Unintentional carbon monoxide poisoning kills between 400 and 500 people per year in the United States. The only form of protection is a carbon monoxide alarm.

"[Carbon monoxide] is undetectable to human senses. You cannot see it, you cannot smell it, and you cannot taste it, so you do not know you've been poisoned until you get sick and start getting headaches, vomiting or pass out," Hampson explained.

Twenty-five states require residences to have these alarms, but 10 of these states now allow exemptions for homes that have no internal sources of carbon monoxide. Many experts are concerned that these exemptions will lead to an increase in accidental poisonings, particularly in multi-family dwellings, where walls between homes are shared.

To prove that carbon monoxide can go through walls, researchers placed varying thicknesses of drywall in a Plexiglas container to observe how quickly the gas could travel through the walls. Because the pores in the wallboard are 1 million times larger than a carbon monoxide molecule, the gas passed easily through the porous barrier. Painted drywall slowed down the gas only a bit.

Only alarms can detect carbon monoxide gas once it is in a home, but far too many homes either don't have one or have one that isn't functioning because the batteries have died or have been removed. According to Lavonas, only 30 percent of American homes have a working carbon monoxide alarm. In North Carolina, a state that has a law requiring the devices, only 67.8 percent of homes do, according to a study published in the American Journal of Public Health in 2012.

There have been many cases of poisonings in homes where carbon monoxide alarms were found, "but they either had no battery in them or they hadn't even been taken out of the package," Hampson said. In addition to changing the batteries regularly, it's important to check the expiration date on the alarm itself, he added.

"When you change your batteries, you should look at the back of the alarm to see when the expiration date is. It's either five or seven years, depending on the manufacturer," Hampson explained.

Unlike smoke alarms, carbon monoxide alarms may be placed anywhere, from the bottom of the wall to the ceiling, and only one is needed per level, preferably located just outside the sleeping areas.

Some alarms can be plugged directly into an electrical outlet or hard-wired, but both Hampson and Lavonas caution that if these are used, they should have a battery back-up. Most carbon monoxide poisonings occur during blackouts, when power is out, they noted.

If your alarm sounds, leave your home immediately, and call the fire department.